Idss RF amplifier

ABSTRACT

A single-power-supply Idss-bias RF amplifier is disclosed, which is composed by the first and second stages Amplifier. The two stages have the same circuit topology except the matching circuits. The source terminal of amplifier is grounded directly to reduce the parasitic effect from the bias circuit. It will increase the stability of the RF amplifier and avoid the oscillation. The lossy matching circuits and eliminating resonator circuit are designed to make the RF amplifier unconditional stable. A variable resister is put into to adjust the D.C. voltage on drain terminal. The current of amplifier could be controlled in reasonable range. High gain and quit low noise have been obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a RF amplifier, and more particularly to a I_(DSS) (saturated drain to source current) RF amplifier adapted to a communication system.

[0003] 2. Description of the Related Art

[0004] Traditional military RF frequency amplifiers are made by thin film technology. This expensive technology makes RF amplifiers become high cost components. Because of the mass-production requirement, most RF amplifiers of the modern communication systems are manufactured on printed circuit boards to reduce costs. The penalty is oscillation resulted from the parasitic effect of bias circuits. There are two ways widely used to supply DC power in designing a RF amplifier, dual-bias circuit and self-bias circuit. The former makes the design of RF amplifier complex, and the later enhances oscillation easily.

[0005] To improve the gain and sensitivity, usually the RF amplifier is used as a low noise amplifier (LNA) which determines the noise figure of the whole receiver. The LNA is the most essential and important component in receiver system.

[0006] To achieve larger gain, in general, the RF amplifier includes two stages of amplifiers. It causes more power consumption and system instability. To design a RF amplifier with simple bias circuit but high stability, it is a quit big challenge for microwave engineers.

[0007] Accordingly, the prior art amplifier of the communication system has following disadvantages:

[0008] 1. Because of the parasitic effect of the resistor and the capacitor of bias circuit, noises and oscillation are easy to be generated in a self-bias amplifier of receiver.

[0009] 2. The prior art amplifier usually uses dual-bias which requires a DC to DC converter to generate negative supply bias.

SUMMARY OF THE INVENTION

[0010] Therefore, an object of the present invention is to provide a I_(DSS) RF amplifier, which can avoid the prior art oscillation effect and reduce the complexity and the cost of RF amplifier.

[0011] The present invention discloses a I_(DSS) RF amplifier, which is adapted for a receiver of a communication system. The amplifier is composed by two stages, the first and the second stage amplifier. The first stage amplifier has a first transistor with the source terminal grounded. The second stage amplifier has the same topology as the first stage except matching circuits.

[0012] According to the preferred I_(DSS) RF amplifier of the present invention, the gate terminal of the first transistor is grounded by the first RF choke. The drain terminal of the first transistor is grounded by series of the fourth resistor and the first capacitor. At the same drain terminal, the first resistor in series with the second RF choke is connected to DC power supply. At the node between the first resistor and the second RF choke, the second capacitor is grounded. The output port of the first stage amplifier is connected to a third capacitor and the third resistor.

[0013] According to the preferred I_(DSS) RF amplifier of the present invention, the gate terminal of the second stage amplifier is ground by the third RF choke. The drain terminal of the second stage amplifier is grounded by series of the fifth resistor and the fourth capacitor. At the same drain terminal, the second resistor in series with the fourth RF choke is connected to DC power supply. At the node between the second resistor and the fourth RF choke, the fifth capacitor is grounded. The drain terminal of the second stage amplifier is connected to the next circuit by the sixth capacitor.

[0014] The I_(DSS) RF amplifier of the present invention uses I_(DSS) as bias current, so the gate of transistor is connected to ground by the first RF choke. There is no resistor existed between the source terminal and ground. It reduce the parasitic effects of the resistor to avoid the unwanted oscillation that the prior art often generates. Moreover, the first stage amplifier is connected to the second stage amplifier through the third resistor which acts as an attenuator to reduce the gain of amplifier and increase the stability.

[0015] In order to make the aforementioned and other objects, features and advantages of the present invention understandable, a preferred embodiment accompanied with figures is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic diagram of I_(DSS) RF amplifier of the present invention.

[0017]FIG. 2 is a measured S parameter of the preferred embodiment of the present invention.

[0018]FIG. 3 is a measurement of noise figure of the preferred I_(DSS) RF amplifier of the present invention.

DESCRIPTION OF SOME EMBODIMENTS

[0019]FIG. 1 is a schematic diagram showing a preferred I_(DSS) RF amplifier of the present invention. Referring to FIG. 1, the microwave amplifier comprises the first stage amplifier 300 and the second stage amplifier 400. The first stage 300 has a transistor 302 with the source terminal of the transistor 302 grounded. The second stage amplifier 400 has a transistor 402 with the source terminal of the transistor 402 grounded.

[0020] According to the preferred I_(DSS) RF amplifier of the present invention, the gate terminal of the transistor 302 of the amplifier 300 is grounded by a RF choke 306. The drain terminal of the transistor 302 is grounded by series of the resistor 308 and capacitor 310. At the same drain terminal, the resistor 304 in series with RF choke 312 is connected to DC power supply Vc. The node between the resistor 304 and the RF choke 312 is grounded by the capacitor 314. The drain terminal of the first stage amplifier 300 is connected to the gate terminal of the next stage by a capacitor 316 and the resistor 350.

[0021] According to the preferred I_(DSS) RF amplifier of the present invention, the transistors 302 and 402 could be high frequency field effect transistor, such as MESFET and HEMT.

[0022] According to the preferred I_(DSS) RF amplifier of the present invention, the input terminal of the second amplifier 400 is grounded by the RF choke 406. The drain terminal of the transistor 402 of the second stage amplifier 400 is grounded by series of a resistor 408 and a capacitor 410. At the same drain terminal, the resistor 404 in series with the RF choke 412 is connected to the DC power supply Vc. The node between the resistor 404 and the RF chock 412 is grounded by a capacitor 414. The drain terminal of the transistor 402 is connected to a capacitor 416.

[0023] According to the preferred I_(DSS) RF amplifier of the present invention, the resistor 308 and the capacitor 310 of the first stage amplifier 300, and the resistor 408 and the capacitor 410 of the second stage 400 are lossy matching circuits, so as to increase the stability of the RF amplifier.

[0024] According to the preferred I_(DSS) RF amplifier of the present invention, the resistor 304 and the capacitor 314 of the first stage amplifier 300, and the resistor 404 and the capacitor 414 of the second stage 400 are designed to eliminate the resonator effect which generated by RF chock 312 and RF chock 412 to make the RF amplifier unconditional stable.

[0025] According to the preferred I_(DSS) RF amplifier of the present invention, the power supply Vc, is about 5V. The V_(DS) (drain to source) voltage of the first stage amplifier 300 can be adjusted by the variable resistor 304. Similarly, the V_(DS) voltage of the second stage amplifier 400 can be adjusted by the variable resistor 404 to improve S/N ratio and control the drain current.

[0026] According to the preferred I_(DSS) RF amplifier of the present invention, the first stage amplifier 300 is connected to the second amplifier 400 by the resistor 350. The resistor 350 works as an attenuator to reduce the gain and avoid oscillation.

[0027]FIG. 2 is a measured S parameter of the preferred embodiment of the present invention. FIG. 3 is a measured noise figure of the preferred I_(DSS) RF amplifier of the present invention. The RF amplifier is realized by a 20-mil printed circuit board (dielectric constant 3.38). Its working range is from 5.2-6.0 GHz. The gain of single stage is over 13 dB around 5.8 GHz. The gain of whole amplifier is over 25 dB and the noise figure is lower than 2.5 dB around 5.8 GHz.

[0028] Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be constructed broadly to include other variants and embodiments of the invention which may be made by those skilled in the field of this art without departing from the scope and range of equivalents of the invention. 

What is claimed is:
 1. A I_(DSS) RF amplifier, comprising: a first stage amplifier having a first transistor, a source terminal of the first transistor is grounded; and a second stage amplifier having a second transistor, a source terminal of the second transistor is grounded, wherein, the first and the second stage amplifier have the same topology except matching circuits.
 2. The I_(DSS) RF amplifier of claim 1, wherein a drain terminal of the first transistor is grounded by series of a fourth resistor and a first capacitor.
 3. The I_(DSS) RF amplifier of claim 2, wherein a gate terminal of the first transistor is grounded by a RF choke.
 4. The I_(DSS) RF amplifier of claim 3, wherein the drain terminal of the first transistor further coupled to a first resistor, the first resistor in series with a second RF choke are connected to a DC power supply, and a node between the first resistor and the second RF choke is grounded by a second capacitor.
 5. The I_(DSS) RF amplifier of claim 4, wherein a output terminal of the first stage amplifier is connected to a gate terminal of the second transistor by a third capacitor and a third resistor.
 6. The I_(DSS) RF amplifier of claim 5, wherein the first resistor is a variable resistor to adjust the voltage of the drain terminal of the first transistor.
 7. The I_(DSS) RF amplifier of claim 6, wherein the first transistor and the second transistors are high frequency field effect transistor, such as MESFET and HEMT.
 8. The I_(DSS) RF amplifier of claim 7, wherein the gate terminal of the second transistor is grounded by the third RF choke.
 9. The I_(DSS) RF amplifier of claim 8, wherein a drain terminal of the second transistor is grounded by a fifth resistor and a fourth capacitor.
 10. The I_(DSS) RF amplifier of claim 9, wherein the drain terminal of the second transistor further coupled to a second resistor, the second resistor in series with a fourth RF choke are connected to the DC power supply, and a node between the second resistor and the fourth RF choke is grounded by a fifth capacitor.
 11. The I_(DSS) RF amplifier of claim 10, wherein the drain terminal of the second transistor is connected to a sixth capacitor which is blocking capacitor. 